1. Field of the Invention
The present invention generally relates to computer networks and, more particularly, to offloading network and file services.
2. Description of the Related Art
The primary task of web and application servers is typically to retrieve files from local file systems and send the file contents to remote clients, for example, over established Transmission Control Protocol (TCP) connections. In a typical application, a remote client connected to a server over a company Intranet or the Internet may request a file from a server. As an example, a web server may receive a request for a file (e.g., a document, image, HTML file, or the like) from a client connected via a TCP socket connection, retrieve the file, and send the file to the requester.
FIG. 1 illustrates such a file access involving a conventional server system. An operating system (e.g., Linux, Windows, or some other operating system) usually segregates the available system memory (and other resources) into user space 110 and kernel space 120. The kernel space is typically reserved for running the kernel, device drivers and any kernel extensions and includes file systems, network interface services, and the like. On the other hand, user space includes memory areas used by user applications. User applications cannot typically access the kernel space directly and, similarly, kernel code cannot access the user space without first checking whether the page is present in memory or swapped out.
As illustrated, when a request for a file is received by an application server 122, the application server 122 typically issues several read requests to copy data from the file system buffer cache 130 in kernel space 120 to buffers in user space 110. For example, in the kernel, the requested data may be first retrieved (e.g., from disk) and copied into the file system buffer cache 130 and then copied into memory the application server 122 can access, such as a buffer created by the application server 122 for this purpose and specified with the read request.
After copying the data into user space, the application server 122 sends the data back out to buffers 140 (e.g., TCP socket buffers) in kernel space 120. In this typical scenario, the application server 122 never needs to actually examine the data. Thus, copying the data from the file system buffer cache 130 to user space is inefficient, resulting in processing delays and consumption of resources (processing and memory resources) in user space 110.
In an effort to more efficiently transfer data, mechanisms, such as the Linux sendfile command, have been created that attempt to avoid inefficient copying of data to and from user space. Rather than copying read data from the file system buffer cache 130 into user space 110, only to send it back down, the application server 122 may issue a single sendfile command specifying a descriptor of a file from which data is to be read from (as well as a location within the file and number of bytes to read), and a descriptor of a socket on which the file will be sent. Because this transfer occurs within the kernel, a copy to and from user space of the application server may be avoided.
Unfortunately, this approach still consumes kernel resources, requiring copying data into the kernel file system buffer cache, writing out to socket buffers, and is less than optimal. As an example, in virtual machine systems running multiple operating system images, for security reasons, care must be taken to ensure each operating system image cannot access memory allocated by the other operating system images. As a result, copying data into kernel space (e.g., to process a sendfile command) typically entails mapping physical memory pages into the operating system image virtual address space. This mapping is typically a lengthy, processing intensive task involving access to operating system image page tables.
Therefore, what is needed is a more efficient mechanism for transferring data from a server, for example, that reduces the impact on local and kernel space resources.